An international research team has found new evidence of how the climate affected ancient societies. The study, published in the journal Science, reveals how periods of climatic instability often coincided with turbulent times in European history. The study was funded in part by MILLENNIUM and ACQWA, two projects supported under the EU's Sixth and Seventh Framework Programmes (FP6 and FP7) respectively. MILLENNIUM ('European climate of the last millennium') received more than EUR 12 million from the 'Sustainable development, global change and ecosystems' Thematic area of FP6, while ACQWA has clinched almost EUR 6.5 million from FP7's Environment Theme.

Led by the Institute for Forest Growth at the University of Freiburg in Germany and the Swiss Federal Research Institute (WSL), the team was made up of climatologists, geographers, archaeologists and historians. By assessing ancient tree rings from more than 7 000 sub-fossil, historical and living tree samples, they were able to reconstruct the history of central Europe's summer temperature and precipitation over the last 2 500 years, rather than the 1 500 years generally used in past studies.

The team then compared variations in European summer climate with human historical events and episodes such as plagues, migrations and the Thirty Years War. Their conclusions shed new light on how climate change played a crucial role in agrarian wealth and economic growth.

'Climate variations have influenced the agricultural productivity, health risk and conflict level of preindustrial societies,' the authors of the study write. 'Discrimination between environmental and anthropogenic impacts on past civilisations, however, remains difficult because of the paucity of high-resolution palaeoclimatic evidence. Here we present tree ring-based reconstructions of central European summer precipitation and temperature variability over the past 2 500 years. Recent warming is unprecedented, but modern hydroclimatic variations may have at times been exceeded in magnitude and duration.'

The researchers point out that the climate data stored within the trees enabled them to compare natural precipitation and temperature fluctuations with the development of European societies. They found that Europe's summer climate during the Roman era, for instance, was relatively warm and wet, and changed little. Increased climate variations from around 250-600 AD, say the researchers, coincided with the fall of the western Roman Empire and the havoc of the Migration Period, during which the population in Europe underwent a major restructuring.

Furthermore, they found that humid and mild summers paralleled the fast political and cultural growth of Medieval Europe, and that a poor climate could have influenced the health conditions that played a part in triggering the economic crisis that emerged during the Black Death plague pandemic in the 14th century.

The researchers note that the temperature minima in the early 17th and 19th centuries coincided with both the settlement abandonment during the Thirty Years' War and the mass migration of many Europeans to America.

'Wet and warm summers occurred during periods of Roman and Medieval prosperity,' the authors write. 'Increased climate variability from around 250-600 AD coincided with the demise of the Western Roman Empire and the turmoil of the Migration Period. Historical circumstances may challenge recent political and fiscal reluctance to mitigate projected climate change,' they add.

The team advises that the projected global climate change may have a much more significant impact on human societies than what researchers currently believe. Moreover, complex causal links between past climate changes and human responses need further research, they say.